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Thursday, March 16, 2017

The Nor'Easter of March 14: What are its Lessons for the Weather Community?

On March 14th, a storm rapidly developed off the Carolina Coast, quickly moved northward, crossing eastern Long Island and SE New England before exiting the region. It produced very strong winds (gusts reaching 40-60 mph over the region), heavy snow (1-2 feet) in the interior, and a nasty slushy mix over the coastal zone. (see snow total map from WeatherBell, Inc.)

In some ways this storm was a great success, a a major event predicted nearly a week out, with verifying strong winds, and snow for many. But in major coastal areas, blizzard warnings and predictions of 1-2 feet did not pan out. As a result, some in the media and a few major politicians were critical of the National Weather Service, who they claimed suppressed changing information on Monday that suggested a lesser coastal event.

In this blog, I will examine this storm and note that it reveals some major issues with modern weather prediction. Issues that are general and reflect problems we had with the Northwest windstorm bust of October 2016.

As noted note above, this forecast could be considered a great success for modern weather prediction, with a real heads up 4-5 days ahead of time. Here are the NWS official surface analyses at 5 AM and 1 PM on Tuesday, March 14th (sea level pressure shown). The low (central pressure of 986 hPa) rapidly intensified (to 978 hPa) as it moved to the tip of Long Island)

The NOAA/NWS GFS forecast valid 5 AM on Tuesday made Friday (4AM), 96 hours before the event, had the right idea and stuck to it (shading is lower atmosphere temperatures). Based on these forecasts, airlines and others started taking pre-emptive action, including suggesting that travelers change their reservations.

Virtually all the major modeling systems (NOAA GFS, European Center, UKMET office, Canadian CMC) honed into a similar solution, with minor (but important) differences in track and intensity.

The implications for the Northeast U.S. were serious. Such a strong storm, with attendant large pressure gradients, would certainly produce strong winds over the Northeast. With cool air over the continent, substantial snow was in the offering for many folks. Where heavy snow and winds came together, blizzard conditions (heavy snow, greater than 35 mph winds, low visibility) would occur.

Blizzard conditions

But there was in issue. A big one. The storm was sweeping warm air into it (see red colors above) aloft and the water temperatures are relatively warm. Thus, there would be a transition zone between snow inland and rain offshore, where a wintry mix of snow, sleet, freezing rain, and rain would occur. The fabled rain-snow line. And clearly the line would be somewhere along or near the coast. If it was just offshore, the NY Metro area would have an historic and crippling blizzard. If it was just along the coast, NY could get a slush storm or rain.

And predicting this line is not easy. It is dependent on the movement of different temperature air around the storm. It is dependent on the intensity of vertical air motions (which can influence temperature). It is dependent on the amount of precipitation (since the evaporation and melting of precipitation can change the temperature profile). It is dependent on coastal effects (where temperature and surface variations are large). Bottom line: the rain-snow line can be a real challenge for even the best models and forecasters. And in a coastal area, it is critically dependent on the exact track and intensity of a storm. And an inability to diagnose and prediction physical details like cloud processes.

On Friday and early in the weekend, most of the model guidance suggested that the rain-snow line would be just offshore, indicated New York and Boston would have mainly a snow event, with the most probable amounts being 1-2 feet. This guidance included high resolution deterministic forecasts and lower-resolution ensembles of many forecasts (used to define uncertainty).

On Sunday, the NWS SREF (Short-Range Ensemble Forecasting System) showed some uncertainty for the total snow forecast at New York's JFK airport (see below), with an ensemble average of around 12 inches and a range from an inch to 29 inches.

The ensemble-based probabilities of precipitation type (below), were dominated by snow (blue lines)

But by Monday morning, the situation had changed substantially. The morning (8 AM) run of the NWS SREF ensemble had decreased the snow to about 10 inches, with more of the ensembles going for lower amounts.

The probability of rain (green) was gaining on snow (blue) and was higher than snow at 11 AM (18 UTC) Tuesday.

The 2 PM run of the SREF went much further, suggesting an event that would start as snow and then transition to rain (see below)

But on Monday, we were closer to the event and forecasters had powerful, more modern tools available.

The future of forecasting such events is high-resolution ensembles, using convection-allowing grid spacing (like 3-km). The National Weather Service does not have such an ensemble system (and it should!), but the National Center for Atmospheric Research (NCAR) does (but a small one with only 10 members). And NCAR ran such an ensemble starting 5 AM on Monday. The average of the ensemble (the ensemble mean) showed a huge coastal snow gradient, with 6-10 inches over Long Island, with heaviest amounts to the west and north. But a slight shift would have huge impacts on NY snow. This kind of situation suggest large uncertainty.

The NCAR ensemble showed lot of uncertain for JFK airport, with a mean of around 8 inches (see below) and a spread from 2-17 inches. It also indicated a change from snow to rain (not shown)

And now the problem. Here is the National Weather Service forecast released 4 AM Monday, one that uses their new approach to presenting uncertainty. The suggested a most probable value for JFK airport of 17 inches and a range from 8 to 22. This was the forecast that was in place for most of Monday morning when a lot decisions were being made. And in addition, there was a winter storm warning and blizzard warning in place over New York on Monday. Nothing like a blizzard warning to get media juices going.

And as uncertainties in the forecast were increasing, the NWS doubled down on the blizzard warning:

By late on Monday, the models shown above were clearly edging towards a lesser event and shorter-range rapid refresh models like HRRR was becoming available (HRRR is initialized every hour and run out for 18 hr). HRRR uses very high resolution (3-km grid spacing), is initialized with lots of regional assets, and the snow output makes use of variable density snow, which gives more accurate totals.

Here is the 18h HRRR snow total starting 5 PM Monday, March 14th, which encompasses pretty much the whole storm in NY. 3-6 inches over the eastern end of LI and the immediate south shore of LI, increasing to the NW, with perhaps 12-15 inches would be expected over the NW side of NY city. Roughly 10-11 inches around JFK.

Still too much, but the pattern is very good. A new version of HRRR, called HRRRx, does even better, with the suggestion that better physics descriptions helps with the overprediction of snow.

Now I have been fixated about snow in the above discussion. Winds were important as well, and the models and the NWS forecasts were very realistic about their strength and duration.

So what is the bottom line of all this? In many ways this was a very successful forecast that shows how far weather forecasting technology has come.

1. The threat of a major Nor'Easter was identified 5-7 days in advance.
2. The large scale prediction was quite accurate, but there were minor but important track and structural errors.
3. The winds were forecast quite well.
4. The general structure of the snowfall was handled reasonably, but the rain-snow line was displaced too far to the southeast by the models, resulting in an overprediction of snow over NY and Boston.
5. There was considerable evidence of forecast uncertainty in the days before, with the possibility of less snow becoming more evident on Monday, March 14. In particular, it become clear by Monday afternoon that a transition from snow to rain was probable over LI and much of NY City.
6. Forecasters held on to the heavy snow/blizzard forecasts on Monday and probably should have backed off that forecast by Monday afternoon, highlighting the change to rain more.

So what are take home messages?

1. The NWS must continue to improve its forecast technology, including high-resolution ensembles that provide uncertainty and probabilistic guidance. The NWS has been delaying too long in building a convection-allowing operational ensemble system. Congress needs to intervene if necessary.

2. NWS forecasts have to transition to a probabilistic framework, where probabilities are given for various outcomes. The old style watch-warning system is really not effective in a new world of uncertainty and probabilistic prediction.

3. Forecasts much continuously evolve as more or better information comes in. Forecasters should not try to second guess users or keep intense forecasts in place to encourage "right" decision making.

15 comments:

Intensive analysis, but surely you are kidding when you suggest the Republican controlled Congress should intervene on behalf of anything related to the improvement of science?

And Trump seems to view weather forecasting as if it's the same field as climate change, and seems hell-bent on defunding anything related to either, along with many other science programs. He doesn't even want a science advisor, that tells you everything you need to know right there.

I think the problem -- you mention it in #2 -- is that we need to train users to accept probabilistic forecasts. But this can be done! And many of the most crucial users, such as school districts, can really use this information to their benefit. "There is a 25% chance of 10+ inches and a 75% chance of 4+ inches" is actually useful to them.

Agreed that convection-permitting ensembles will help a lot of the needs of the weather community, but in this case: (a) were proper modeling of nonhydrostatic motions key to providing a good forecast? I'm not sure. The success of the GFS many days out suggests that the essential dynamics were largely hydrostatic, baroclinic. (b) With not unlimited computer power, we have to limit the domain size for limited-area nonhydrostatic models. It's quite possible that the eastern edge of the domain would have cut through the storm circulation, at least assuming a fixed domain. And (c) I'd key in on getting the low-level temperature structure right, which is likely due more to sufficient resolution in the lower levels of the atmosphere and getting physics right, such as microphysics, land-surface temperatures, and so forth.

Thanks for the in depth analysis. I was supposed to fly into Philadelphia on Tuesday so I was watching the forecast pretty closely leading up to the storm and that included reading the forecast discussion for the area. The forecast discussion did include those nuances but they did not get transferred to the general forecast. I've noticed that in the PNW as well. Thus your comment about more probalistic forecasting is spoot on. Based on the discussion I changed my flight to Wednesday which is a good thing because even though the flight from Seattle to Philadelphia took off, albeit late, my connecting flight was cancelled.

The Europeans and Canadians forecast much better than we do.... I'd leave it to them why spend the money.... we've been spending it and we still come in 3rd place. Please for once leave politics out of it--- something tells me it won't be.

Well, there's always politics involved and Congress has a lot to say about NOAA/NWS spending. There's also a move afoot to privatize the Air Traffic Control system. The National weather Service will be next. Just imaging pating cash for your forecast.

And Trump seems to view weather forecasting as if it's the same field as climate change, and seems hell-bent on defunding anything related to either, along with many other science programs. He doesn't even want a science advisor, that tells you everything you need to know right there.

We've seen a great deal of corruption of the meteorologists by the global warming cult, so it's understandable. If the meteorologists would get back to their knitting and not be props for a cult, maybe this wouldn't be happening.

It's very clear that we're way behind in our weather infrastructure, and we have a president that wants to reduce our national debt. My thought, mind you it just popped into my head, why not let everybody else invest a ton of money into the tech for a few years, while we proactively fund for an explosion of weather technology equipment in a few years after the private sector and other countries pour a ton of time and research into doing what we can't seem to do correctly anyway? Just a thought

My point is that we need a much larger and diverse ensemble system to move towards a better probabilistic approach. Getting to 3-km would allow explicit convection, which would help in many situations. And in many situations, convection and convective bands are very important. It would also allow better definition of the land-ocean boundary, which is important in such coastal regions. You can see some of the benefits in the HRRR simulations. More vertical resolution is useful...but not as useful as one might think (I have done a large amount of experiments with this). Better micro certainly would help...one of the reasons HRRRx did better. The domain must be large enough to extended sufficiently off the coast(few hundred miles at least). Although unlimited computer power is never available, this is clearly within reach with a modest investment by the nation....cliff

Thank you for a great summary of the forecast issues associated with this storm. Here in New York there is another piece to this story. Governor Cuomo said he wanted to avoid a spat with the forecasters but still managed to insult them and deflect any criticism away from his administration’s actions because the forecast was “not correct”.

I watched him on a news conference where he went out of his way to say that he did not want to say the forecast was wrong but then proceeded to say what happened was not what was forecast. He also said I don’t want to upset forecasters by saying the forecast was wrong. Only a politician could claim that the forecasters should not be upset because of what I said about their forecast because I did not say it was wrong.

Meanwhile, in the real world on Saturday I talked to a friend of mine who works for the Syracuse area NYS Dept. of Transportation. He said that he was going to be sent to Long Island on Sunday to prepare for the storm. It is clear to me that Cuomo (well I would hope his administration but given his tendency to micro manage he probably made the decision) needs to talk to a forecaster when they are making their plans. Based on the status of the forecasts on Saturday going all the way down to Long Island was an error on their part that could have been avoided by talking to a forecaster who could explain the situation. The particular problem is that getting to Long Island means you have to go through New York City which is a bottleneck in the best of times and could easily result in getting caught in a traffic jam.

I think we are on the same page for the advice to the State prior to the storm. Had I been asked I would have pointed out that this was a tough forecast to be exact and a little change in the path would make a huge difference in who got the biggest impact. We knew there would be a coastal storm, it was going to be worse in places where there was no switch over to rain, but exactly where that happens in New York was going to range between Montauk and Kingston. We wouldn’t know where until Monday, best case early in the day but possibly late in the day. Therefore the State should make plans based on that uncertainty.

My advice would have been to stage the plow crews just north of New York City where they could go to the aid of Long Island and New York City if they got hit, would already be in place for the lower Hudson Valley if they got hit or back home to upstate if they got hit. But to send them all the way to Long Island was, in my opinion, more pandering to his voting base than doing the best thing based on the evidence at hand.

I agree with your take home messages but I don’t think they will resolve all the problems. Ultimately there is interpretation involved and someone who understands the requirements and forecasts has to provide the decision maker relevant information to make the best decision. I guess that is the role of the private forecasting companies because they should work with clients to be sure they understand exactly what is needed so they can tailor the forecast recommendations.

Awesome blog!Really, this was a well organized piece. I agree that the modeling was showing signs 5-6 days out. And the GFS, I think, outperformed the EURO. The EURO at 5AM Tuesday still was showing 17 inches for the Philly area, with hardly any mixing in sight. We got 4.5 in Wilmington, DE. We were scheduled for at least 12. Yet, at 11pm that night the HRRR was showing a significant ice/mixing event. And it was the NAM, that was amazingly consistent. At 10pm it was showing the storm push further north along the coast, then ride the coast giving the big cities STILL big amounts but hinted that the storm for the big cities was a bust. Yet, to get .20 inches of ice and 3 inches of snow, and sleet was not seen by a lot of people. I think the NOAA should have caught it. It still was a significant storm.